Atrial fibrillation and ventricular tachyarrhythmias occurring in patients with structurally abnormal hearts represent the most frequently encountered tachycardias, and account for most of the morbidity and mortality from tachyarrhythmias. Atrial fibrillation affects over two million people in the United States, increases mortality two-fold, and causes over 75,000 strokes per year. Since drug therapy is usually unable to suppress atrial fibrillation, and since significant side effects occur, the surgical-maze procedure has been studied as a non- pharmacologic approach for suppressing atrial fibrillation, while maintaining atrial function. This procedure involves creating multiple linear incisions in both atria, which create lines of conduction block, thereby compartmentalizing the atrium into distinct segments that cannot sustain atrial fibrillation. The maze procedure can restore normal rhythm, but has not gained widespread acceptance because of the long duration of recovery and the risks of cardiac surgery. Attempts at applying RF catheter ablation to mimic the surgical-maze procedure in a minimally-invasive setting have been attempted. The success of catheter ablation has been limited, however by the long time duration of procedures, resulting from the difficulty of creating continuous linear (surgical-like) lesions in a setting where areas of ablated myocardium cannot be directly visualized. The applicants hypothesize that MRI, with intracardiac receiver antennas can visualize ablation lesions, which should greatly simplify production of continuous linear lesions. In this proposal, the investigators will develop novel catheters that are a combined MRI receiver antenna, electrophysiology measuring device, and RF ablation unit. They will also develop software to display ablated myocardium in three dimensions and apply these techniques to atrial fibrillation ablation. In addition to guiding ablation of atrial fibrillation, the use of MRI may also allow: 1) quantification of atrial function to provide an assessment of future risk of embolization; 2) integration of concomitant treatment strategies to reduce the extent of ablation needed; and 3) study of the effects of different RF ablation modes in real time non-invasively. Thus, this atrial fibrillation ablation approach is a paradigm for a new approach to electrophysiology, which may eliminate the need for ionizing radiation while increasing diagnostic and therapeutic capabilities, and may be directly applicable to many other procedures, both in cardiology as well as other fields.